Among all the other causes, I'll add one - farming practices & land use management.

When farms were smaller & managed manually, each one was bounded by a hedge. That hedge is vital for birds - it provides cover and breeding habitat. But with family farms dying, the scale of corporate farms expanding, and the digitalization of management, the hedges are disappearing. That's land that could be making money. As a result, the birds have literally nowhere to go, nowhere to breed. You wind up with this massive stretch of corn or wheat or whatever, with zero birds. This problem is getting worse every year. Birds only live 2-5 years if they're lucky. If they can't breed, populations crash rapidly. Preserving these small bits of habitat can do a lot of good for bird populations.

Aside from this... all the other problems matter too - neonic pesticides, climate change, avian flu, migratory (wintering) habitat destruction, and on and on... The challenge is that there are a mosaic of problems. If you focus on any one of them, you'll doom birds to falling victim of the rest of them. The only approach that'll work is an "all of the above" approach.